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Monday, November 5, 2012

Understanding Climate Change Part 5 - Ocean Circulation and Climate

Ocean Circulation and Climate

(Reading #5 for my course on Climate Change, Alan Holyoak, PhD)

Daily Objectives

1.     Be able to explain the relationship between subtropical high-pressure cells, the Coriolis Effect, and the major ocean surface currents.
2.     Be able to explain why the Gulf Stream plays a particularly important role in the climate of Western Europe.
3.     Be able to explain why water from the Gulf Stream sinks.
4.     Be able to explain how the Gulf Stream drives the global conveyor.
5.     Be able to explain why different masses of water in the deep sea do not readily mix with each other.
6.     Be able to explain how Ekman transport pulls deep water to the surface.

Introduction

            The climate is the product of complex interactions between solar radiation, the atmosphere, land, and ocean.  You learned about radiation and the atmosphere in earlier readings and discussions.  Now it it’s time to focus on the role of the ocean in the Earth’s climate.

Ocean Currents

            Heat from solar radiation is absorbed by surface waters of the oceans, and is then transported around the planet in ocean currents.  These currents fall into two main categories: 1) surface currents, and 2) deep-water currents.  We have known about and monitored surface currents for hundreds of years, but deep-water currents were discovered only relatively recently, and we are still learning about them and their affect on climate. 

Surface Currents

            The world’s major surface currents are wind-driven.  Surface winds circulate around the subtropical high-pressure regions.  These high-pressure regions exist as persistent cells in each major ocean basin, and are the result of air returning to the surface at the boundary between Hadley and Ferrel Cells.  Figure 1 shows the ITCZ and the locations of the high-pressure cells as they typically appear in July.  Also recall that air flows from high-pressure regions toward low-pressure regions.
A helpful way to think about high and low pressure regions and the flow of air between them is to imagine that a high-pressure region develops when dry air piles up on the surface and forms a mound or hill of air.  A low-pressure region, on the other hand, can be imagined as a valley or depression in the atmosphere next to the surface because air there tends to be moist and less dense, thus rising away from the surface. So you can imagine air that is on the top of a high-pressure cell (hill) will flow “downhill” toward a neighboring low-pressure cell “valley”. 
Don’t forget the Coriolis Effect!  This effect causes moving air to deflect to the right in the northern hemisphere and toward the left in the southern hemisphere.  The combination of the movement of air between high and low pressure cells and the Coriolis Effect produces prevailing wind currents indicated by the arrows on the map in Fig.1.  This combination of air movement and Coriolis Effect gives rise to winds that generally circulate in a clockwise direction in the northern hemisphere and a counterclockwise direction in the southern hemisphere.
Figure 1. The ITCZ (red line near the equator), and regional high-pressure and low- pressure cells, and prevailing surface winds during the month of July.  The size and direction of small arrows indicate prevailing wind direction and speed. (Image: Modified by Dr. Hipps, USU.)

Just like the ITCZ, subtropical high-pressure cells move north and south with the seasons.  Though they vary in strength with the seasons, winds around them are quite persistent (Fig. 2).  Because surface winds are persistent, they blow across the ocean surface, create friction, and produce surface ocean currents. 
Figure 2. Seasonal shifts in the location and strength of high-pressure cells.  

Clearly the major currents in each ocean basin surround subtropical high-pressure cells, one per ocean basin (Fig. 3).  Cold-water currents are indicated in blue and warm-water currents are indicated in red.  Currents shown in black do not move very far north or south.  When a current moves away from high latitudes it moves cold water toward the equator, and thus providing a heat sink for equatorial heat (i.e., a place where heat is removed from the atmosphere).  Conversely, when a current moves from lower latitudes toward higher latitudes, it moves warm water toward the poles and is a heat source for higher latitudes.  In each case heat is being transferred, and the movement of water impacts regional and global climates. 

Figure 3. Prevailing major ocean currents of the world.  Currents that carry warm water away from the tropics = red.  Currents that carry cold water toward the tropics = blue.  (Image: Arnold J. Bloom, UC Davis.)

There is a cold-water current running along the west coast of every continent and a warm-water current running along the east coast of every continent (Fig. 3).  This reflects the direction of wind currents as they flow around subtropical highs.  You should also note that these major surface currents routinely pass under boundaries between the atmospheric Polar, Ferrel, and Hadley Cells, thus moving heat away from the tropics to higher latitudes.
The cold current off of the west coast of North America is most pronounced during the northern hemisphere summer when the Pacific subtropical high strengthens and moves closer to North America.   Cold water from this current produces low clouds and cool temperatures during summer months along most of the west coast of the Pacific Northwest and much of California, producing cool, moist, temperate rainforest climate. 
The Gulf Stream is particularly important to global climate.  The Gulf Stream originates in the Caribbean Sea, carries warm water northward along the east coast of the United States, flows across the North Atlantic Ocean, and eventually bathes Western Europe with that warm water.  Because of this, Europe’s climate is much warmer than its latitude would suggest.  Think about this for a minute: Rexburg, Idaho, is located at about 44oN latitude, and London, England, is located at about 51oN latitude.  This means that London is nearly 400 miles farther north than Rexburg.  To get that far north you would have to drive to Lethbridge, Alberta, Canada!  This means that if everything else were equal London should be colder than Rexburg, but it’s not.  London is actually cooler in the summer and warmer in the winter than Rexburg (Fig. 4).  OK, back to currents and climate.

 
Figure 4. The average monthly temperatures and monthly average rainfall in Rexburg, Idaho (left), and London, England (right). (Image: weather.com.)

Benjamin Franklin made the first map of the Gulf Stream.  He had heard reports of a river of warm water in the Atlantic Ocean, so during his many trips between North America and Europe he made many temperature readings at various locations.   His map, based on his temperature measurements, was presented to the British in 1769.  His original map is readily comparable to a modern satellite thermal image of the Gulf Stream (Fig. 5).  Franklin’s map is amazingly accurate.  This just goes to show that you don’t always have to have lots of high-tech equipment make good observations and reach powerful conclusions.
Take notice of the defined edges of the Gulf Stream.  This current retains its integrity as it travels across the Atlantic Ocean due to density differences between the warm Gulf Stream water and colder water of the North Atlantic Ocean.  This density difference restricts mixing between the water masses.  The Gulf Stream and most other major currents therefore retain a high degree of internal consistency, including the retention of heat, as they move through different latitudes.

 
Figure 5. Benjamin Franklin’s map of the Gulf Stream (left: Wikimedia Commons) and a thermal satellite image of the Gulf Stream (right: NASA).

            If anything were to slow the rate of the Gulf Stream the climate of Western Europe would at the very least cool, and at the most affect the global climate by changing the speed of deep-water currents.

Deep-water Currents

            Unlike surface currents, deep-water currents are not driven by the wind.  Instead, the movement of water into and back up from the deep sea is caused by something called thermohaline circulation.  That is, because of differences in seawater density related to temperature and salinity.  Everyone is familiar with temperature, but salinity may be a new concept for some.  Salinity is the term that refers to the salt content of water.  All water, even freshwater, contains some salt, but seawater contains significantly more.  About 3.5% of seawater is made up of salt ions, on average, though the amount of salt varies slightly from region to region (Fig. 6). 
Water density increases when water temperature drops, salt content increases, or both.  However, increasing salt content makes density grow larger than temperature changes can.  This is because water becomes less dense than liquid water when it freezes.  Water is the only common, naturally occurring compound that does this, and this characteristic allows life as we know it to exist, but that is topic probably better left for another time.  OK, back to deep-water currents.
 
Figure 6. Surface seawater salinity of the world’s oceans.  PSU = Practical Salinity Units, and equals the amount of salt in seawater in parts per thousand.  So 34 PSU = 34 parts per thousand salt or 3.4% salt by weight. Note that water in the Caribbean Sea and the Gulf Stream are more saline than most other surface waters.  (Image: Wikimedia Commons.)

            Deep-water circulation of the oceans is caused and driven by density differences of water.  The process that drives global-scale deep-water currents is called thermohaline circulation.  This term refers to the combination of temperature and salinity that creates the movement of water because of its density.  The deep-water current that is driven by this process is sometimes called the conveyor belt, the global conveyor, or the Atlantic conveyor (Fig. 7). 
The Atlantic contribution to the global conveyor is driven primarily by a unique set of conditions that exist in the North Atlantic Ocean.  Seawater in the North Atlantic is particularly cold, about -2oC or 28oF, and sea ice forms there.  About now some of you might be saying, “Hey, wait a minute, water freezes at 0oC or 32oF!”  That’s true, unless that water has a lot of material dissolved in it, like salt.  The more material water has dissolved in it, the colder it has to be before it will freeze.  This is called freezing point depression.  Anyway, when seawater freezes, salt is excluded.  The excluded salt has to go someplace so it is added to the salinity of water nearby that didn’t freeze. This water is consequently saltier, and denser than it was before.  In the meantime, water that is more saline than average sweeps northern in the Gulf Stream.  This water undergoes evaporation as it flows north and its salinity also increases.

Figure 7. The Atlantic or global conveyor system.  Dense water sinks in the North Atlantic Ocean as well as in the Antarctic.  This water flows along the ocean floor until it resurfaces in the North Pacific and the Indian Ocean.  Once back at the surface, water moves across the world’s oceans until it reaches the North Atlantic or the Arctic again and it sinks.  (Image: Wikimedia Commons.)

Thermohaline circulation occurs when salty water from the North Atlantic and the Gulf Stream experience further evaporation and becomes saltier, cools, and becomes dense enough to sink.  Water also sinks in the Antarctic region where super-cooled water becomes salty as sea ice forms, and that hypersaline, super-cooled water sinks (Fig. 7).  The depth to which a mass of water sinks depends entirely on the density (temperature and salinity) of water masses around it.  It turns out that the movement of water in the deep ocean is a much more complex process than we originally imagined (Fig. 8 and 9), but it wasn’t until recently that scientists realized the important role of the global conveyor to global climate. 
Figure 8. Movement of masses of deep water in the Atlantic Ocean (top), Pacific Ocean (middle), and Indian Ocean (bottom).  Note that there are distinct masses of water separated from each other by density differences determined by temperature and salinity.  Denser water masses are closer to the bottom, and less dense water is closer to the surface.  Also note that the mass of water from the Red Sea is fairly deep.  Though temperature is not shown, it is a warmer water mass than masses around it, but it is hypersaline, thus very dense.  (Image: Nybakken and Bertness, 2004.)

Figure 9. A more detailed view of sources and temperature/salinity characteristics of deep water in the Atlantic Ocean.  Note that North Atlantic Deep Water and bottom water from Antarctica have the same salinity, but Antarctic water is denser due its lower temperature. (Image: Wikimedia Commons.)

Thermohaline Circulation and Climate
           
            In recent years we have become more aware of the role of thermohaline circulation in the global climate.  There is compelling evidence that in past ice ages thermohaline circulation stopped.  This would reduce the transport of heat to higher latitudes.  Some concern has arisen about how the current trend of global warming might affect the Atlantic conveyor system.  We already know that the largest warming has been and will continue to be observed in the northern high latitudes.  This means the sea ice and land glaciers will continue to melt, and freshwater will be released into the North Atlantic.  The resulting reduction in salinity in the North Atlantic could reduce the salinity of surface waters.  This could slow circulation, since it is the sinking of salty water that drives the process.
            In fact, data already show that salinity in the North Atlantic region has been decreasing.  At the present time, the best estimates are that circulation will slow somewhat in response to continued warming, but not stop.  This could possibly cause some regional cooling.  At present it takes water between 1000-1600 years to complete one circuit of the global conveyor.  Of course, more research needs to be conducted before a definitive prediction can be made.  For now, we simply note that the thermohaline circulation is an important factor in global climate change. 
            Why is the global conveyor an important climate factor, aside from moving or not moving heat?  When water from the surface sinks, water is not the only thing that sinks.  Sinking water carries whatever is in it.  This includes plankton and dissolved gases.  Plankton is made of organic materials – including carbon.  So plankton and any other organic material that sinks removes carbon from the surface of the earth.  It is hypothesized that much of this carbon is deposited at the bottom of the ocean where it is trapped in ocean sediments.  This is what is referred to as a carbon sink.  Dissolved CO2 is also removed from the water as deep sea organisms, including animals and small single-celled foraminiferans and radiolarians take it up to secrete calcium carbonate (CaCO3) shells (Fig. 10).  This is also part of the oceanic carbon sink.  Plus, seawater absorbs a huge amount of CO2 directly from the atmosphere.

 
Figure 10. Scanning electron micrograph of shells of marine foraminiferans, a type of single-celled marine organism that secretes calcium carbonate shells. (Image from Wikimedia). 

Upwelling

            Another important form of water movement is referred to as upwelling.  Upwelling occurs when surface currents move along continental margins and Coriolis Effect and winds push surface waters away from the coast.  When this happens, deep water is pulled to the surface to replace it.  The way upwelling was discovered is pretty interesting.
            At the beginning of the 20th Century, scientists were puzzled about why icebergs did not move in the direction of the wind or surface currents.  Instead icebergs moved at an angle from the prevailing wind/water direction.  One of the scientists gave the problem to a graduate student whose name was Ekman.  He assembled the proper equations describing flow that contained Coriolis Effect and friction.  He then solved them to show how water flow changes with depth.  The resulting description is now known as Ekman Transport or the Ekman Spiral.
            A simple way of visualizing the process is to consider that the flow is being deflected with depth.  Since most of an iceberg (91% of it, actually) is below the surface, subsurface effects have a greater effect on it than wind or effects observed right at the water surface.  It turns out that water flow below the surface does not move not in exactly the same direction as surface water.  In fact, to understand the Ekman Effect you need to think about water as being in a number of layers stacked on top of each other.  The surface layer is driven mainly by friction due to wind energy, but the Coriolis Effect causes the surface layer to deflect slightly to the right in the northern hemisphere.  The water layer just below the surface also moves by friction with the layer above it, and it too deflects to the right relative to the layer above it.  This friction reduces the amount of energy available to move the layers farther below, but what we observe is that eventually you will see that once you reach a certain depth the direction of water movement is 180o opposite that observed at the surface (Fig. 12).
            So, as water flows along a shoreline, subsurface layers move away from the coast by the Ekman Spiral, and deeper water is pulled toward the surface (Fig 11).  Figure 12 also shows why this is referred to as the Ekman Spiral.  You see the spiral when you connect the ends of the arrows representing flow.

Figure 11.  This figure shows Ekman Transport of deeper water toward the surface, as it would occur in the southern hemisphere.  How do you know it’s in the south? (Image: Dr.  Hipps, USU.)

Figure 12. The Ekman Spiral showing changes in the direction and rate of motion of layers of water with increasing depth.  The stronger the wind, the deeper the Ekman Spiral will extend.  (Image: Wikimedia Commons.)

As a result of upwelling, colder deeper water rises to the surface.  So upwelling makes surface temperatures colder.  These lower temperatures also affect the climate of coastal regions where upwelling regularly occurs.  Figure 13 shows regions of coastal upwelling.  Upwelling brings not only colder water to the surface, but also brings nutrients to the surface.  This makes areas with upwelling biologically productive and economically important as fisheries – not that this has anything to do with climate, but if climate changed in a way that affected surface winds and currents, then upwelling would be affected too.

 
Figure 13. Map showing regions of coastal upwelling. (Jmage: Dr. Hipps, USU.)

            The most important locations for upwelling are along the west edges of North and South America, and Africa.  Look back at Fig. 3 and you will see that currents in these regions are flowing from high toward low latitudes, and are cold currents.  The upwelling makes surface waters even colder, and has a large effect on the climate of these regions in moderating warm temperatures.
            For example, the cold California Current flows south along the coast of the Pacific Northwest and much of California during the summer.  This is a result of the subtropical high in the Pacific intensifying and moving closer to North America.  The current experiences upwelling, making surface waters cold.  Low clouds and cool temperatures characterize the coastal climates during summer in this region.  Without this current, the region would be much warmer, and this would impact regional climates in terms of temperature and precipitation.

Source material

Hipps, LE. 2010. Personal communication and readings produced by Dr. Hipps. Professor of Atmospheric Science, Department of Plants, Soils, and Climate.  Utah State University.

Nybakken, J.W., and M.D. Bertness. 2004. Marine Biology: An Ecological Approach, 6th edition.  Pierson/Benjamin Cummings Press.

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